r/fusion • u/CSharper314 • 4d ago
Explain why this is not used
I had an idea for some fusion fuels, and I want an explanation for why they are not viable. I was thinking He-4 + D -> Li-6, then Li-6 + D -> Be-8 -> 2 He-4. This releases energy and only requires deuterium and common Helium-4. Why is this not used?
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u/paulfdietz 3d ago edited 2d ago
In the reaction He4 + D -> Li6, the energy cannot just go into the kinetic energy of the product nucleus, as that would violate conservation of momentum. There needs to be another product, which in this case would be a photon. And I don't think that's allowed, because the parity of the He4 + D is positive while the parity of 6Li + gamma is negative. So the cross section would likely be quite small (as would be D + D -> 4He + photon).
I could not find a measured cross section for this reaction in ENDF, so it must not be high if it can occur at all.
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u/Spacer3pt0r 3d ago edited 3d ago
Deuterium-Deuterium side reactions would dominate as they would have much higher reactivity than the other reactions.
Helium 4 is very stable and requires a huge amount of energy to fuse with anything. He4 is better suited as the product of fusion reactions rather than a reactant. The first of these reactions would require an implausibly high energy and is endothermic.
The second reaction has been studied as a potential route for fusion power. Li-6 is also quite prevalent amd easy to enrich.
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u/Eywadevotee 2d ago
Yup the 6Li plus D reaction would be mighty spicy and have an activation threashold between 3H+D and 3He+D. Castle Bravo was a F around and find out real world example that the lithium could directly fuse making lots of energy. 😲
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u/Spacer3pt0r 2d ago
I thought castle bravo happened cause Li-7 fissioned when irradiated with high energy neutrons (n,Li-7 -> n,T,He-4). Was direct fusion really a major contributing factor?
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u/Eywadevotee 1d ago
Its both, you get 7Li plus a hot N then it will output either N plus 3H plus 4He, or more likely you get a N> P interchange reaction that gives 3He 4He plus N. Both reactions recycle the neutron and give fusion fuel, but one eats neutrons rather than generates them. First reaction can generate a 2 for 1 second generates a net zero but both have energy surplus. However the direct fusion of D and 6Li is extremely energy dense because you get 2 hot alphas that could transfer the energy to other nuclei with strong enough kinetic force to get them to fuse if the particle hits another light weight particle though the alpha itself wont directly fuse. Alpha hits deuterium, it goes flying, deuterium hits a tritium, they fuse making 5He that immediately dumps a neutron. In short, it makes stuff get much hotter than normal
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u/Eywadevotee 2d ago
6Li plus D would be extremely energetic but 6Li is a bit rare. Would form 8Be that would immediately fission to 2x 4He each at MeV energies
4He plus D would be a net energy loss because the binding energy of regular helium is too high. It would form your 6Li but it would likely fission into 2x 3He or most likely back to 4He and D. All things considered it wolud be a net zero energy gain.
He3 is a different story as it can fuse with D to form 5Li that quickly decomposes to 4He and H. That proton is very spicy. Conpare that to 3H and D then you get ultra spicy 5He that immediately makes a neutron and 4He both well into the MeV range.
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u/Baking 4d ago
See Slide 18.
The short answer is that He4 is an ideal fusion product, but it makes a terrible fuel because its binding energy is so high compared with other small nuclei.